A fume hood is a ventilation device used to minimize exposure to hazardous or noxious fumes, vapors or dust for laboratory workers. Referring to FIG. 1A, a configuration for a fume hood 10 is shown. The fume hood 10 is located in a lab area or room and includes a cabinet structure 12 having a hood opening 14 and a work area 16 located within the fume hood 10. The hood opening 14 may be defined by a cutout in a front side or other side of the cabinet structure 12 having a vertical or longitudinal dimension of “y” and a horizontal or latitudinal dimension of “x”, or sash width x, as shown in FIG. 1A. A movable, transparent sash 18 is used to open or close the hood opening 14. In one embodiment, the sash 18 moves in a vertical direction such that the sash 18 is spaced above an edge 19 of the work area 16 to form a sash opening 20 (as portion of hood opening 14) having a sash height 17 (within the range of 0 to “y”) for providing access to the work area 16. The fume hood 10 is connected to an exhaust fan and damper arrangement by ductwork (not shown in FIG. 1A or 1B). The exhaust fan serves to draw air from the room through the sash opening 20, work area 16, ductwork and the damper. The air is then vented outside of the building by the exhaust fan thus removing fumes, vapors or dust. In an alternate embodiment (see FIG. 1B), the fume hood 10 includes a sash 22 that moves either vertically or horizontally relative to the hoop opening 14 to form a sash opening 24.
It is desirable to maintain the speed of the air (i.e. the face velocity) drawn through the fume hood 10 within a desired air speed range. If the face velocity is too low, there may be insufficient venting of the work area 16. If the face velocity is too high, undesirable air turbulence is generated which may cause movement of the contaminants into a worker's breathing zone. An acceptable range for face velocity may vary between approximately 80-120 feet per minute (fpm) depending on the type of hood and hazard. The face velocity may be determined based on an area or size of the sash opening 20 and a pressure drop value across the sash 18 measured by pressure sensors.
In order to maintain the face velocity within the desired range, the speed of the exhaust fan and/or associated damper must be adjusted to take into account the current size of the sash opening 20. In particular, the speed of the exhaust fan is increased or a damper opening is increased as the size of the sash opening 20 is increased. Conversely, the speed of the exhaust fan is decreased or the damper opening is decreased as the size of the sash opening 20 is decreased. Similarly, the speed of the exhaust fan and/or associated damper must be adjusted to take into account the size of the sash opening 24 for the configuration shown in FIG. 1B.
Therefore, it is desirable that the size of a sash opening be accurately and reliably obtained so that the speed of the exhaust fan and the damper are properly adjusted and the face velocity is kept within the desired range.